[PATCH 3.19 093/101] [media] vb2: fix UNBALANCED warnings when calling vb2_thread_stop()

Greg Kroah-Hartman
2015-04-17T13:44:52Z

From: Hans Verkuil <hverkuil@xs4all.nl>

commit 0e661006370b7e7fb9ac9d94f9c3500a62cd559b upstream.

Stopping the vb2 thread (as used by several DVB devices) can result
in an 'UNBALANCED' warning such as this:

vb2: counters for queue ffff880407ee9828: UNBALANCED!
vb2: setup: 1 start_streaming: 1 stop_streaming: 1
vb2: wait_prepare: 249333 wait_finish: 249334

This is due to a race condition between stopping the thread and
calling vb2_internal_streamoff(). While I have not been able to deduce
the exact mechanism how this race condition can produce this warning,
I can see that the way the stream is stopped is likely to lead to a
race somewhere.

This patch simplifies how this is done by first ensuring that the
thread is completely stopped before cleaning up the vb2 queue. It
does that by setting threadio->stop to true, followed by a call to
vb2_queue_error() which will wake up the thread. The thread sees that
'stop' is true and it will exit.

The call to kthread_stop() waits until the thread has exited, and only
then is the queue cleaned up by calling __vb2_cleanup_fileio().

This is a much cleaner sequence and the warning has now disappeared.

Reported-by: Jurgen Kramer <gtmkramer@xs4all.nl>
Tested-by: Jurgen Kramer <gtmkramer@xs4all.nl>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/media/v4l2-core/videobuf2-core.c | 11 +++--------
1 file changed, 3 insertions(+), 8 deletions(-)

--- a/drivers/media/v4l2-core/videobuf2-core.c
+++ b/drivers/media/v4l2-core/videobuf2-core.c
@@ -3230,18 +3230,13 @@ int vb2_thread_stop(struct vb2_queue *q)

if (threadio == NULL)
return 0;
- call_void_qop(q, wait_finish, q);
threadio->stop = true;
- vb2_internal_streamoff(q, q->type);
- call_void_qop(q, wait_prepare, q);
+ /* Wake up all pending sleeps in the thread */
+ vb2_queue_error(q);
err = kthread_stop(threadio->thread);
- q->fileio = NULL;
- fileio->req.count = 0;
- vb2_reqbufs(q, &fileio->req);
- kfree(fileio);
+ __vb2_cleanup_fileio(q);
threadio->thread = NULL;
kfree(threadio);
- q->fileio = NULL;
q->threadio = NULL;
return err;
}